Nanocatalytic Materials for Energy-Related Small-Molecules Conversions: Active Site Design, Identification and Structure-Performance Relationship Discovery.

Abstract

The catalytic conversion of energy-related small-molecules is a critical process in the fields of chemical production, environmental restoration, and energy conversion and storage. Over the years, numerous nanocatalytic materials have been explored in efforts to substantially boost the inherently sluggish catalytic processes. Despite achievements, the lack of fundamental insights into the design and identification of active sites and the structure-performance relationship has been one of the main obstacles to further improvement in catalytic performance. With the development of first-principles density functional theory (DFT) calculations and state-of-art spectroscopic techniques, the pace of research has started to move forward again.In this Account, we illustrate our recent representative attempts to gain fundamental insights into the rational development of efficient nanocatalytic materials and thus boost the typical electrochemical and mechanochemical conversions of energy-related small-molecules, including for the hydrogen evolution reaction (HER), oxygen reduction reaction (ORR), and ammonia synthesis. DFT calculations and advanced spectroscopic techniques, such as synchrotron radiation-based X-ray absorption spectroscopy (XAS, hard and soft X-ray), were properly adopted for this purpose.Specifically, to achieve a fast-electrochemical hydrogen evolution process, Ir active sites with balanced hydrogen adsorption/desorption behaviors were first computationally designed via orbital modulation and experimentally identified, and they showed significantly enhanced catalytic activity toward HER in acidic media. For the electrochemical reduction of oxygen, well-designed Zn-N2 active sites and quinone functional groups were introduced into the different carbon matrixes and structurally identified by the XAS technique, utilizing hard and soft X-rays, respectively. Both experimental and DFT studies revealed that Zn-N2 active sites with their unique structure can greatly activate the adsorbed oxygen species, leading to a highly efficient and selective four-electron oxygen reduction pathway, while the quinone functional groups are able to modify the activation mode and alter it into a selective two-electron oxygen reduction pathway for H2O2 production.In another study, inspired by the dissociation of stable nitrogen molecules on the surface of Fe, dynamic strained Fe active sites were designed for mechanochemical ammonia synthesis. Combined XAS and Mössbauer spectroscopy revealed the formation of a short-range Fe4N structure by the Fe active sites and dissociated nitrogen during the ball milling process, facilitating robust hydrogenation and ammonia production under mild conditions.Thanks to the theoretical methods and advanced spectroscopic techniques, fundamental insights into the design and identification of active sites and understanding of the structure-performance relationship can be easily obtained using such tools, which will guide the development of nanocatalytic materials and boost the conversions of energy-related small-molecules for various applications.